Flowerlike Co3O4 microspheres loaded with copper nanoparticle as an efficient bifunctional catalyst for lithium–air batteries

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Abstract

Porous flowerlike Co3O4 microspheres/Cu nanoparticles composite has been synthesized via a combined solvothermal method, subsequent thermal treatment and polyol process. Due to the 3D mesoporous structure, the resulting Co3O4 microspheres/Cu catalyst shows an efficient and stable bifunctional catalytic activity. The cobalt oxide-based catalysts show better performance during the discharging and charging processes at a current density of 0.05 mA cm 2 compared with that of the Vulcan XC-72. The cell with this novel catalyst can be reversibly charged/discharged and has a good cycle performance. The preliminary results indicate that the Porous flowerlike Co3O4 microspheres/Cu nanoparticles composite is a promising material for a metal/air battery or a PEM fuel cell as an efficient and stable bifunctional catalyst.

Highlights

► Prepare a porous micro/nanostructured composite material consisting of flowerlike Co3O4 microsphere and Cu nanoparticles. ► Study on the catalytic behavior of this novel catalyst in a hybrid electrolyte system. ► Demonstrate its potential application in a lithium-air battery as an efficient and stable bifunctional catalyst.

Introduction

The high cost of energy storage and conversion devices such as the proton-exchange-membrane (PEM) fuel cells and metal/air batteries restrains their practical use [1], [2]. Among various metal/air batteries, lithium–air batteries possess the highest theoretical gravimetric energy density. However, for rechargeable lithium–air battery, another issue that has to be addressed in the current technology is the limitations of oxygen reduction reaction (ORR) during discharging process and oxygen evolution reaction (OER) during charging process. The sluggish kinetics of ORR and OER in lithium–air batteries are ascribed to the low efficiency of catalysts [3]. The performance of Li–air batteries can be drastically improved by incorporating an efficient catalyst to achieve higher discharge voltage, lower charge voltage and rate performance [4]. Therefore, the design of a low-cost and stable bifunctional electrocatalyst is a major challenge to the construction of efficient Li–air batteries.

Many spinel cobaltite oxides have been investigated as electrocatalysts for the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) [5], [6]. Molecular mechanisms involving O2/H2O cycles at cobalt centers suggest the involvement of Co2 +, Co3 +, and likely Co4 + oxidation states during catalysis [7]. In an oxygen-atom ligand field, Co2 + (t2g5eg2) is a high spin ion and substitutionally labile, whereas Co3 + (t2g6eg0) with a higher oxidation state is low spin and substitutionally inert [8]. In general, the ORR is postulated to take place at active sites associated with the cations at the oxide surface in a higher oxidation state [6].

Bruce et al. reported a screening of many catalysts that could be used in facilitating the electrochemical properties of the O2 electrode in a non-aqueous Li/O2 cell [2]. Among the oxide catalysts studied, Co3O4 gives the best compromise between initial capacity and capacity retention as well as the lowest charging voltage of 4 V. Dai et al. reported a hybrid material consisting of Co3O4 nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the ORR and OER [9]. Recently, we demonstrated perovskite Sr0.95Ce0.05CoO3  δ loaded with copper nanoparticles on their surface are shown to be excellent, low-cost, and stable bifunctional catalysts for oxygen-reduction and oxygen-evolution reactions in aqueous solution [10]. Very recently, Xu et al. demonstrated that the ORR catalytic activity of the prepared Co3O4-based catalysts are sensitive to the number and activity of surface-exposed Co3 + ions that can be tailored by the morphology of cobalt oxides [6]. Porous Co3O4 microspheres with an open mesoporous structure have more exposed Co3 + species and can increase dispersion of another active component [11]. In this work, we examine and compare the ORR and OER activities of porous Co3O4 microspheres and compare with the Vulcan XC-72 and the 50% Pt/carbon-black catalysts. To further improve the catalytic performance, copper nanoparticles were deposited on the surface of the porous Co3O4 microspheres. The preliminary results show that a round-trip electric-energy storage efficiency of 75.7% with excellent long-term stability and high rate performance was obtained in an aqueous Li/air cell with the Co3O4 microspheres–Cu catalyst.

Section snippets

Experimental

The flowerlike Co3O4 microspheres were synthesized by a hydrothermal method as reported elsewhere [11]. Copper nanoparticles were loaded on the surface of the porous Co3O4 microspheres by a polyol method [10].

Scanning electron microscopy (SEM) was performed on a Quanta 650 scanning electron microscope. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were carried out at a working voltage of 200 kV (FEI Tecnai-F20). The copper loading was

Results and discussion

The size and morphology of the Co3O4 were examined by a field-emission scanning electron microscope (FESEM). Fig. 1a shows that most of the sample displays monodispersed spherical particles with flowerlike texture. The diameter of the microspheres is in the range of 2–5 μm. It can be clearly seen that these flowerlike microspheres are composed of many nanoplate petals with an average thickness of about 50 nm; these nanoplates interweave together forming an open porous structure (Fig. 1b). A HRTEM

Acknowledgments

This work is financially supported by the National Science Foundation of China (NSFC) (Grant Nos. 51172275, 11004229), the National Key Basic Research Program of China (Grant No. 2012CB215402), and the Institute of Physics (IOP) start-up funding for the talents. Y. Kim's work is supported by Research Support Funds Grant (RSFG) at Indiana University Purdue University Indianapolis (IUPUI). This paper is dedicated to Prof. John B. Goodenough on the occasion of his 90th birthday.

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